Anyone used a cocoonababy?

I am using one for my son who is 14 weeks. The nest has been amazing and he sleeps really well in it. However, I feel I need to wean him off it as he is almost too big for it so would rather try and get him out it sooner rather than later.

My problem is he won't sleep on just his mattress and wakes up after an hour of going to bed which he has never done when asleep in the cocoonababy nest.

I have tried rolling towels up and placing under fitted sheet to try and make him feel more secure but that hasn't made any diff.

My question is- has anyone used one if these great mattresses and if so how did you get your baby to make the transition to just sleeping on his normal mattress?